• Question: hey how do we know that light travels in a striaght line?

    Asked by bobchelsea to Mark, Matthew, Mike, Paul, Sabina on 13 Mar 2014.
    • Photo: Paul Coxon

      Paul Coxon answered on 13 Mar 2014:

      Imagine a single source of light, like a lightbulb. The light emanates from the bulb as a wave surrounding the bulb in all directions. We say that light moves in straight lines because it helps us understand the movement of the wave. The motion can be thought of as a pebble in a calm pond. Dropping the pebble sets up a series of ripples that over the surface of the water, spreading away in a circular pattern from where the pebble was dropped.

      If you watch a spot on a ripple, you’ll see it starts at the centre, moving away in a straight line as the ripple spreads out. This is how light moves. It’s a wave, but tracking the path of the wave shows it travels in a straight line.

    • Photo: Mike Lee

      Mike Lee answered on 13 Mar 2014:

      Because we can’t see around walls!

    • Photo: Mark Jackson

      Mark Jackson answered on 14 Mar 2014:

      In day-to-day life we know that light goes in a straight line because if you trace out the path it takes, there are no shorter paths. Geometrically this means the path is straight. But under certain circumstances – for example, when light in the air hits water – the light doesn’t go straight, but instead bends. The reason for this is known as the Least Action Principle, which I explain here http://goo.gl/eSnlSu.

      There are also instances in which gravity makes light curve. Einstein’s General Theory of Relativity shows that gravity is just the curvature of space and time, and light follows the straightest path *in the curved spacetimeMATOMO_URL This was precisely how we confirmed General Relavity using the experiment described at http://goo.gl/0G7Cqv. But this might seem like a paradox – how can you take the straightest path in a curved space? You already know a simple example of this: if you pick two points on the surface of the Earth, what is the shortest path between them? You would just walk (or swim) directly from one to the other, even though the Earth’s surface is curved. If you kept walking you would make a large circle. In more sophisticated examples there is a prescription for determining which curve is shortest, known as a “Geodesic.” This branch of mathematics is called Riemannian geometry and is extraordinarily useful in physics.

Comments